The invention relates to determining the resistivity of geological formations through which a well provided with metal casing passes.
The importance of resistivity logging for oil prospecting is well known. It is known that the resistivity of a formation depends essentially on the fluid that it contains. A formation containing salt water, which is conductive, has resistivity that is much lower than a formation filled with hydrocarbons, and therefore resistivity measurements are of irreplaceable value for locating hydrocarbon deposits. Resistivity logging has been performed widely and for many years, in particular by means of apparatus having electrodes, but existing techniques have a field of application that is limited to wells that are not cased, or xe2x80x9copen holesxe2x80x9d as they are referred to in oil industry terminology. The presence in the well of metal casing, which has resistivity that is minute compared with typical values for geological formations (about 2xc3x9710xe2x88x927 ohm.m for steel casing compared with values in the range 1 ohm.m to 1000 ohm.m for a formation), represents a considerable barrier to sending electrical currents into the formations surrounding the casing. As a result, in particular, it is not possible to obtain resistivity measurements in wells that are in production, since such wells are provided with casing.
It would therefore be very advantageous to make it possible to measure resistivity in sections of cased wells. Such measurements, taken in a well in production at the level of the deposit, would make it possible to locate the water-hydrocarbon interfaces and thus to monitor how the positions of such interfaces vary over time, so as to monitor the behavior of the hydrocarbon reservoir and so as to optimize extraction therefrom. It would also be possible to obtain a resistivity measurement in a well (or a well section) for which no measurements were taken before the casing was put into place, in particular in order to supplement knowledge of the reservoir, and optionally to detect productive layers that were not located initially.
Proposals have been made on this subject in the literature. The basic measurement principle presented in U.S. Pat. No. 2,459,196 consists in causing a current to flow along the casing under conditions such that current leaks out or is lost to the formation. Such loss is a function of the resistivity of the formation: the more the formation is conductive, the greater the current loss. By measuring current loss, it is possible to determine the resistivity of the formation. Current loss can be evaluated by measuring the voltage drop between electrodes placed at different depths in the well. U.S. Pat. No. 2,729,784 describes a measurement method using two pairs of measurement electrodes a,b and b,c spaced apart along the casing, the electrodes a and c being in principle equidistant from the electrode b. Current electrodes are placed on either side of the measurement electrodes so as to inject currents into the casing in opposite directions. A feedback loop servo-controls the injected current so as to put the external measurement electrodes at the same potential, so as to cancel out the effect of the resistance of the casing varying in the sections (a,b) and (b,c) as defined by the measurement electrodes. A value for the leakage current at the level of the central electrode b is obtained by measuring the voltage drop across each of the pairs of electrodes a,b and b,c, and by taking the difference between the voltage drops, which difference is stated to be proportional to the leakage current.
French Patent Document 2 207 278 provides the use of three measurement electrodes spaced apart uniformly as in U.S. Pat. No. 2,729,784 for measuring current leakage, and it describes a method in two steps: a first step for measuring the resistance of the casing section defined by the external measurement electrodes, during which step current is caused to flow along the casing so that there is no leakage into the formation; and a second step during which current can leak to the formation. For that purpose, a current injection system is provided that comprises one emission electrode and two return electrodes, a near one of the measurement electrodes being active during the first step, and the other measurement electrode being situated at the surface and being active during the second step.
U.S. Pat. No. 4,796,186 describes a method in two steps of the same type as the method described in above-mentioned French Patent Document 2 207 278, and using the same electrode configuration. It provides a circuit for cancelling out the effect of resistance varying between the two sections of casing. That circuit comprises amplifiers connected to each pair of measurement electrodes so as to deliver respective voltage drops at their outputs. One of the amplifiers is a variable-gain amplifier, its gain being adjusted during the first step so as to cancel out the difference between the outputs of the amplifiers. U.S. Pat. No. 4,820,989 describes an identical compensation technique.
Using Ohm""s law, in order to determine the resistivity of the formation, in addition to knowing the leakage current as measured using one of the methods indicated, it is also necessary to know the potential difference relative to infinity of the casing at the measurement level. In the above-mentioned documents, that potential difference is measured by means of a reference electrode situated at the surface, and at a sufficient distance from the above-mentioned surface return electrode.
The use of a reference electrode suffers from operational drawbacks. The corresponding measurement must be taken separately from the above-mentioned measurements, and it thus represents an additional step which increases the total duration of the operations. It also represents a source of error, it being possible for the potential of the reference electrode to be affected by various phenomena. Proposals have been made to omit such a reference electrode. U.S. Pat. No. 5,510,712 proposes applying currents to the casing at two places that are spaced apart in the longitudinal direction. Similarly, U.S. Pat. No. 5,543,715 proposes an additional current electrode. Those proposals suffer from the drawback of complicating the measurement apparatus and in particular of increasing the length thereof.
In one aspect, the invention provides a method of surveying the resistivity of a geological formation through which a borehole provided with metal casing passes, in which method a leakage current is caused to leak into said formation outside the casing, and said leakage current is determined on a casing section at a certain level, the leakage current being indicative of the resistivity of the formation, said method being characterized by the fact that the resistivity is determined on the basis of the leakage current by applying a factor that depends on the distance z between said level and the surface.
In a preferred implementation, said factor takes into account the length of the casing.